Over one hundred years ago, W. C. Roentgen discovered X-radiation by the inadvertent exposure of a silver halide photographic element. In 1913, Eastman Kodak Company introduced its first product specifically intended to be exposed by X-radiation (X-rays). Today, radiographic silver halide films account for the overwhelming majority of medical diagnostic images. Such films provide viewable black-and-white images upon imagewise exposure followed by processing with the suitable wet developing and fixing photochemicals.
In medical radiography an image of a patient's anatomy is produced by exposing the patient to X-rays and recording the pattern of penetrating X-radiation using a radiographic film containing at least one radiation-sensitive silver halide emulsion layer coated on a transparent support. X-radiation can be directly recorded by the emulsion layer where only low levels of exposure are required. Because of the potential harm of exposure to the patient, an efficient approach to reducing patient exposure is to employ one or more phosphor-containing intensifying screens in combination with the radiographic film (usually both in the front and back of the film). An intensifying screen absorbs X-rays and emits longer wavelength electromagnetic radiation that the silver halide emulsions more readily absorb.
Another technique for reducing patient exposure is to coat two silver halide emulsion layers on opposite sides of the film support to form a "dual coated" radiographic film so the film can provide suitable images with less exposure. Of course, a number of commercial products provide assemblies of both dual coated films in combination with two intensifying screens to allow the lowest possible patient exposure to X-rays. Typical arrangements of film and screens are described in considerable detail for example in U.S. Pat. No. 4,803,150 (Dickerson et al), U.S. Pat. No. 5,021,327 (Bunch et al) and U.S. Pat. No. 5,576,156 (Dickerson).
One important component of the films described in these patents is a microcrystalline dye located in a silver halide emulsion layer or antihalation layer that reduces "crossover" (exposure of an emulsion from light emitted by an intensifying screen on the opposite of the film support) to less than 10%. Crossover results in reduced image sharpness. These microcrystalline dyes are readily decolorized during the wet processing cycle so they are not visible in the resulting image.
Radiographic films that can be rapidly wet processed (that is, processed in an automatic processor within 90 seconds and preferably less than 45 seconds) are also described in the noted U.S. Pat. No. 5,576,156. Typical processing cycles include contacting with a black-and-white developing composition, desilvering with a fixing composition, and rinsing and drying. Films processed in this fashion are then ready for image viewing. In recent years, there has been an emphasis in the industry for more rapidly processing such films to increase equipment productivity and to enable medical professionals to make faster and better medical decisions.
As could be expected, image quality and workflow productivity (that is processing time) are of paramount importance in choosing a radiographic imaging system [radiographic film and intensifying screen(s)]. One problem with known systems is that these requirements are not necessarily mutually inclusive. Some film/screen combinations provide excellent image quality but cannot be rapidly processed. Other combinations can be rapidly processed but image quality may be diminished. Both features are not readily provided at the same time.
In addition, the characteristic graphical plots [density vs. log E (exposure)] that demonstrate a film's response to a patient's attenuation of X-ray absorption indicate that known films do not generally provide desired sensitivity at the highest image densities where important pathology might be present. Traditionally, such characteristic sensitometric "curves" are S-shaped. That is the lower to midscale curve shape is similar to but inverted in comparison with the midscale to upper scale curve shape. Thus, these curves tend to be symmetrical about a density midpoint.
Another concern in the industry is the need to have radiographic films that as accurately as possible show all gradations of density differences against all backgrounds. It is well known that the typical response of the human eye to determining equal differences in density against a background of increasing density is not linear. In other words, typically it is more different for the human eye to see an object against a dark background than it is to see an object against a lighter background. Therefore, when an object is imaged (for example using X-rays, with or without intensifying screens) at the higher densities of the sensitometric curves, it is less readily apparent to the human eye when the radiographic film is being viewed. Obviously, this is not a desirable situation when medical images are being viewed and used for important diagnostic purposes.
In order to compensate for this nonlinearity of response by the human eye, it would be desirable to somehow increase radiographic film contrast only at the higher densities without changing contrast or other properties at lower densities. The result of such a modification would be a unique sensitometric curve shape where the contrast is higher than normal in the higher density regions. Such a curve shape is considered as providing "visually adaptive contrast" (VAC).
While this type of sensitometry sounds like a simple solution to a well known problem, achieving it in complicated radiographic film/screen systems is not simple and is not readily apparent from what is already known in the art. Moreover, one cannot predict that even if VAC is obtained with a particular radiographic film, other necessary image properties and rapid processability may be adversely affected.
Recent digital technologies in the photographic industry offer advantages in that they can enable the user to manipulate the images after wet processing by scanning to create a digital representation of the image. One of these advantages is the ability to readjust the exposure by automatic tone scaling to correct for either over- or underexposure. This is particularly useful in radiography where a patient is not available to have a second X-ray image taken (for example, the patient may be too ill), as in intensive care facilities. A problem with known digital modalities is that they do not provide the high image quality that high performance film/screen imaging assemblies are capable of.
In addition, while photographic film scanners are available today, and films scanning and digitization is common, existing radiographic films do not work will with known film scanning equipment. One limitation is that the scanned film must have sufficient exposure latitude so that information can be recovered digitally even if the film is over- or underexposed. It is thus necessary that the film exhibits sufficient contrast in the both the toe and shoulder regions of a characteristic sensitometric density vs. log E curve to capture image information.
Early attempts to accomplish this are described in U.S. Pat. No. 4,755,447 (Kitts, Jr.). The films described in this patent may be suitable for recovering information even if over- or underexposed, but their contrast was too low if directly viewed. In addition, films at a high density level are less useful because the image signals are dominated by electronic noise (see for example, Bunch et al, Applied Optics, Vol. 27, No. 16, pp. 3468-3474, 1988).
With these constraints in mind, there is a need for a means to provide high quality images that can be directly viewed as well as electronically scanned and digitized for further manipulation.